(© 2005 Ryota Kanai)
The image is regular at the center, but the grid pattern is less regular at the peripheral parts of the images (both on the left and right edges). As you stare at the center of the grid for say 20 seconds, the regularity of the grid pattern at the center spreads into the irregular parts in the periphery. This illusion seems to indicate the preference of the visual brain to see regular patterns.
Read more about the illusion and possible explanationsHealing Grid,